The present invention relates to electric power meters and, more particularly to a phase canceling current transducer for a power meter.
Electric power is typically generated at a remote, central generating facility and transmitted to the consumer over a distribution grid. To reduce transmission losses, a step-up, sub-transmission transformer is used to increase the voltage and reduce the current for transmission over the transmission lines of the distribution grid. The actual transmission line voltage usually depends on the distance between the sub-transmission transformers and the consumers of the electricity but is commonly in the range of 2-35 kilo-volts (“kV”). Distribution substation transformers and distribution transformers of a utility's secondary power distribution system reduce the voltage from the transmission line level to a distribution voltage for delivery and use by industrial, commercial, and residential consumers. In the United States, for example, electric power is typically delivered to a facility as a 60 Hertz (Hz), alternating current (AC) voltage ranging from 120-660 volts (“V”), depending upon the use.
While the total power consumption of a building or other facility is monitored by the electric utility with a power meter located between the distribution transformer and the facility's power distribution panel, in many circumstances, particularly in business environments, it is desirable to monitor the power consumption of individual loads or groups of loads, such as motors, lighting, heating units, cooling units, machinery, etc. or to sub-meter or attribute the facility's power usage and cost to different occupancies, buildings, departments, or cost centers within the facility. These loads are typically connected to one or more of the branch circuits that extend from the power distribution panel and each may be supplied with single phase or multi-phase power. In addition, it is often desirable to monitor several parameters related to efficient electric power distribution and consumption, such as active power, the time rate of transferring or transforming energy; the apparent power, the product of the root mean square (RMS) voltage and current; and the reactive power, the product of the RMS voltage and the quadrature component of the current. Flexibility has favored adoption of digital power meters incorporating data processing systems that can monitor a plurality of circuits and calculate the desired output parameters.
As generated, the fundamental AC voltage and current of the U.S. power grid approximate in-phase, 60 Hertz (“Hz”) sine waves over time. The effective or true power of the analog sinusoidal voltage and current waveforms is the integral of the product of the instantaneous magnitudes of the voltage and current averaged over a time period, usually a cycle of the waveform:
                    P        =                              1            T                    ⁢                                    ∫              0              T                        ⁢                          (                                                v                  ⁡                                      (                    t                    )                                                  ⁢                                  i                  ⁡                                      (                    t                    )                                                  ⁢                                                                  ⁢                                  ⅆ                  t                                                                                        (        1        )            
where: P=effective or true power (watts)                v(t)=instantaneous voltage at time t        i(t)=instantaneous current at time t        T=time period, typically the period of a waveform cycle        
Referring to FIG. 1, in a digital power meter 20 the effective power is typically approximated by averaging the sum of a plurality of products of the instantaneous amplitudes of the voltage and current that are obtained by sampling the voltage and current waveforms at periodic intervals for a period of time, typically making up at least one cycle of the waveform:
                    P        ≅                              1            T                    ⁢                                    ∑                              k                =                1                                            k                =                                  T                                      Δ                    ⁢                                                                                  ⁢                    t                                                                        ⁢                                                  ⁢                                          v                ⁡                                  (                  k                  )                                            ⁢                              i                ⁡                                  (                  k                  )                                            ⁢              Δ              ⁢                                                          ⁢              t                                                          (        2        )            
where: P=effective power                v(k)=sample voltage for the k-th sample        i(k)=sample current for the k-th sample        Δt=sampling intervalA digital power meter 20 comprises, generally, at least one voltage transducer 22, at least one current transducer 28, voltage and current sampling units 30, 32 and a data processing unit 34 to control the sampling units, read the instantaneous magnitudes of the voltage and current, and calculate the power and other output parameters from respective magnitudes of a plurality of voltage and current samples.        
The voltage transducer 22 is commonly a voltage divider network that is connected to the conductor in which the voltage will be measured. The exemplary power meter 20 includes three voltage transducers 22, 24, 26 each connected to a bus bar 36, 38, 40 in a power distribution panel. Each of the bus bars conducts a single phase of the three-phase power delivered to the power distribution panel from the supply 44, typically the distribution transformer for the facility. The power distribution panel provides a convenient location for connecting the voltage transducers because the voltage and phase is the same for all loads attached to a particular bus bar and interconnection of the transducer and the facility's wiring is facilitated by the wiring connections in the power distribution panel. However, the voltage transducer(s) can be interconnected anywhere in the wiring connecting the supply 44 and the load, including connection at the terminals of a load, for example, terminals 46, 48, 50 of the exemplary 3-phase load 52 or the terminal of the single-phase load 54.
A typical current transducer, for example, current transducer 74, comprises a resistor network 56 and an associated current transformer 58. A current transformer typically comprises a core having a portion defining an aperture and a secondary winding that encircles the cross-section of the portion of the core that defines the aperture. For example, the current transformer 58 comprises a secondary winding 60 comprising multiple turns of conductive wire wrapped around the cross-section of a toroidal core 62. The current transducer 70 includes a split core current transformer. The core of a split core current transformer typically comprises a C-shaped or U-shaped first core portion and a second C-shaped core portion, U-shaped core portion or a bar portion that is arranged to connect the ends of the first core portion. The second core portion is typically hinged to the first core portion or separable from the first core portion to facilitate routing a conductor through the core's aperture without disconnecting the conductor.
The conductor of the current that is to be measured is passed through the aperture in the core and constitutes the primary winding of the transformer. Varying current flowing in the primary winding (primary current) induces a secondary voltage and current in the secondary winding which is connected to the resistor network 56. The magnitude of the primary current is determined from the amplitude of the voltage at the output of the resistor network. The primary winding has N1 turns (typically, N1=1) and the secondary winding has N2 turns and, thus, the current transformer has a turns ratio (n) of N1/N2 and, ideally, the current in the conductor is equal to the product of the current in the second winding and the turns ratio (n).
To measure the power consumed by a load, a current transformer is installed encircling each conductor conducting power to the load (or a conductor of a shunt current representative of the load current). For example, three current transformers 58, 74, 76 are arranged to encircle three conductors 64, 66, 68 connecting the exemplary 3-phase load 52 to the supply 44 and a single split-core current transformer 70 encircles the single conductor 72 connecting the exemplary single-phase load 54 to the supply. (Neutral conductors are not illustrated). The associated resistor networks may be incorporated in the enclosures of the current transformers, for example, enclosure 29 (shown in phantom), and are conductively connected to the power meter.
The digital power metering system includes pluralities of voltage and current transducers and multiplexers 82, 84 sequentially connect respective transducers to respective voltage and current sampling units 30, 32. The sampling units 30, 32 typically comprise an analog-to-digital converter (ADC) including a sample and hold circuit that periodically holds the output voltage of the respective transducer constant and a quantizer that converts the analog output voltage of the transducer to a digital signal. In the sampling units, time, the independent variable of the sinusoidal waveform equation, is converted from a continuum to a plurality of discrete moments and the concurrent magnitudes of the voltage or current transducer signals are converted to discrete, binary values of finite precision. A clock 86, which may be included in the data processing unit 34, provides a time reference enabling the data processing unit to output at least one sampling signal 88 to trigger the sampling of the voltage and current by the respective sampling units 30, 32.
The outputs of the sampling units are read by the data processing unit 34 which, in a typical digital power meter, comprises at least one microprocessor or digital signal processor (DSP). The data processing unit reads and stores the digital values quantifying the magnitudes of the current and voltage samples and uses the values to calculate the current, voltage, power, and other electrical parameters that are output to a display 90 for immediate viewing or to a communications interface 92 enabling transmission to another data processing system, such as a building management computer, for remote display or further processing, for example formulating instructions to automated building equipment. The digital power meter also includes a memory 94 in which operating instructions for the data processing unit, current and voltage samples, and calculated output are stored.
In addition, accurate measurement of electric power requires compensation for error introduced by the transducers of the power meter. For example, the secondary current of a current transformer is ideally equal to the current in the conductor (the primary winding) divided by the number of turns in the secondary winding (turns ratio). However, actual transformers are not ideal transformers and the magnetization of the core of a current transformer causes the primary current to be less than the product of the secondary current and the turns ratio. Current transformer error comprises a phase error and a ratio error and before calculating the current, the data processing system typically adjusts the value of the sensed instantaneous load current to compensate for the effects of phase error and ratio error introduced by the current transformer.
Referring to FIG. 2, the ratio error (re) varies with the magnitude of the primary current (I1) as follows:re(%)=K3+K4(log I1)  (1)                where K3 and K4 are constants.The effect of the ratio error is to alter the relationship between the magnitudes of the measured secondary current (I2) and the primary current (I1) from the theoretical relationship, that is:I1=I2(n)  (2)        where n=turns ratio,to the relationship:        
                              I          1                =                              I            2            ′                    ⁡                      (                          n              +                                                nr                  e                                100                                      )                                              (        3        )                            where I′2=measured secondary currentThe magnitude of the measured secondary current (I2′) is related to the theoretical secondary current (I2), as follows:        
                              I          2                =                              I            2            ′                    ⁡                      (                          1              +                                                r                  e                                100                                      )                                              (        4        )            
Referring to FIG. 3, magnetization of the transformer core and windings also causes a phase shift between the current in the primary winding and the current in the secondary winding. The resulting phase error (P) varies with the magnitude of the primary current (I1) approximately according to the relationship:P=K1+K2(I1−M)  (5)
where M, K1 and K2 are constants
In practice M is often approximately equal to ½ and, consequently, a square root approximation can often be conveniently employed as part of the overall correction algorithm.
The values of the constants K1, K2, K3, and K4 depend upon the configuration of the particular current transformer. Factors such as core material and turns ratio affect the values of the constants which are typically ascertained by experiment with samples of a given core configuration. Typically, the values of K1, K2, K3, and K4 are determined for a particular transformer configuration or production batch by comparing the actual performance of a sample of the transformer configuration to the performance of a standard device when the secondary winding is connected in parallel to a particular impedance or burden. Typically, the error correction factors are generated from a sample of a particular transformer configuration and stored in the memory of the meter's data processing system, often as a table or a mathematical formula relating the error factors to the magnitude of the sensed current. When the current is sampled, the data processing system looks up or calculates the appropriate error correction factors for a current equal to the sensed current and adjusts the magnitude of the sensed current as required by the ratio and phase error correction factors.
Since the phase and ratio errors are peculiar to a particular current transformer or batch of current transformers, maintaining the desired accuracy of the power meter when replacing or adding a current transducer often necessitates reprogramming the meter to update the phase and ratio error correction factors for the new transformer. Updating the error correction factors can be particularly complex when the power meter includes large numbers of current transformers, particularly when the error correction factors are not common to all of the transformers. What is desired, therefore, is a current transducer which can be installed in a power meter without requiring the meter to be reprogrammed to maintain its accuracy.